Proposed research will concentrate on isolation and purification of the serum suppressor substance which accumulates in the serum of mice infected with Trypanosoma cruzi. The purified substance will be injected into rabbits and the subsequent antisera tested for ability to abrogate suppression in vivo and in vitro. Because previous results indicate that the suppressor substance is the mediator of the suppression phenomenon in experimental Chagas' disease, inhibition of this substance with antibody, especially in vivo, may provide insight into the significance of immunosuppression in the protozoan parasitic disease. Other experiments will attempt to identify the cells responsible for the production of the suppressor substance and the mechanism of its action on cells of the immune system.